1. Field of the Invention
The present invention relates to a method of evaluating a deteriorated state of a hydrocarbon adsorbent which is disposed in the exhaust passage of an internal combustion engine for adsorbing hydrocarbons in an exhaust gas emitted from the internal combustion engine.
2. Description of the Related Art
Some known systems for purifying exhaust gases emitted from internal combustion engines have an exhaust gas purifier disposed in the exhaust passage, which may comprise a hydrocarbon adsorbent such as zeolite or a hydrocarbon adsorbing catalyst comprising a composite combination of a hydrocarbon adsorbent and a three-way catalyst, for adsorbing hydrocarbons (HC) in the exhaust gas while the catalytic converter such as a three-way catalyst or the like is not functioning sufficiently, i.e., while the catalytic converter is not sufficiently high in temperature and not activated, such as when the internal combustion engine starts to operate at a low temperature. The hydrocarbon adsorbent functions to adsorb hydrocarbons in the exhaust gas at relatively low temperatures below 100xc2x0 C., for example, and operates to release the adsorbed hydrocarbons when heated to a certain temperature in the range from 100 to 250xc2x0 C., for example.
One conventional technique for evaluating a deteriorated state of such a hydrocarbon adsorbent is disclosed in Japanese laid-open patent publication No. 10-159543, for example. According to the disclosed technique, temperature sensors are positioned respectively upstream and downstream of an exhaust gas purifier which has the hydrocarbon adsorbent. On the assumption that the hydrocarbon adsorbent is not deteriorated but brand-new, the temperature downstream of the exhaust gas purifier is estimated from the temperature detected by the temperature sensor that is positioned upstream of the exhaust gas purifier. A deteriorated state of the hydrocarbon adsorbent is then evaluated on the basis of the difference between the estimated temperature and the temperature detected by the temperature sensor that is positioned downstream of the exhaust gas purifier.
However, it is difficult for the above conventional evaluating system to evaluate a deteriorated state of the hydrocarbon adsorbent with accuracy because the temperatures detected by the respective temperature sensors positioned upstream and downstream of the exhaust gas purifier are susceptible to various factors including the ambient temperature, etc. The conventional evaluating system is disadvantageous as to its cost because it requires two temperature sensors positioned respectively upstream and downstream of the exhaust gas purifier.
According to another known technique, a hydrocarbon sensor (HC sensor) is positioned downstream of a hydrocarbon adsorbent, and an adsorbed state of HC adsorbed by the hydrocarbon adsorbent, e.g., an adsorbed amount of HC, is directly recognized on the basis of a detected output signal from the HC sensor, so that a deteriorated state of the hydrocarbon adsorbent can be evaluated from the recognized adsorbed state of HC.
Generally, however, an HC sensor reacts with not all kinds of hydrocarbons that can be adsorbed by a hydrocarbon adsorbent. Consequently, the adsorbed state of HC adsorbed by the hydrocarbon adsorbent, as recognized by the HC sensor, may not sufficiently represent the actually adsorbed state of HC adsorbed by the hydrocarbon adsorbent. Accordingly, it often is difficult to accurately evaluate a deteriorated state of the hydrocarbon adsorbent. In addition, the HC sensor poses a cost problem as it is relatively expensive.
It is therefore an object of the present invention to provide a method of accurately evaluating, with a relatively inexpensive arrangement, a deteriorated state of a hydrocarbon adsorbent disposed in the exhaust passage of an internal combustion engine.
According to the findings of the inventor of the present invention, a hydrocarbon adsorbent disposed in the exhaust passage of an internal combustion engine is capable of adsorbing not only hydrocarbons contained in the exhaust gas emitted from the internal combustion engine, but also moisture in the exhaust gas. The ability of the hydrocarbon adsorbent to adsorb moisture is highly related to its ability to adsorb hydrocarbons (a maximum amount of hydrocarbons that can be adsorbed). The ability to adsorb moisture and the ability to adsorb hydrocarbons drop in the same manner as each other when the hydrocarbon adsorbent is progressively deteriorated. Therefore, the ability to adsorb hydrocarbons can be evaluated by evaluating the ability of the hydrocarbon adsorbent to adsorb moisture.
As described in detail later on, the findings of the inventors of the present invention indicate that the humidity (relative humidity) in the exhaust passage near the hydrocarbon adsorbent changes after the internal combustion engine is shut off, as follows: When the internal combustion engine is shut off after it has been normally operated continuously for a certain period of time, the humidity (relative humidity) near the hydrocarbon adsorbent increases, as indicated by the left end portions of curves shown in an upper section of FIG. 4 of the accompanying drawings, immediately after the internal combustion engine is shut off (generally for a period of time of several tens minutes from the termination of the operation of the internal combustion engine) because the saturated water vapor pressure drops due to a temperature reduction caused by the radiation of heat from the hydrocarbon adsorbent. After the temperature of the hydrocarbon adsorbent falls to a temperature capable of adsorbing moisture, the hydrocarbon adsorbent starts adsorbing moisture in the exhaust gas that remains around the hydrocarbon adsorbent. Therefore, the humidity (relative humidity) of the exhaust gas near (around) the hydrocarbon adsorbent increases to a maximum value and thereafter decreases. The humidity increases and decreases near the hydrocarbon adsorbent, i.e., the humidity increases before the maximum value and decreases after the maximum value, not instantaneously, but within a period of time ranging from several tens minutes to several hours. When the hydrocarbon adsorbent continuously adsorbs moisture until it is saturated, the humidity near (around) the hydrocarbon adsorbent becomes substantially constant, or more specifically, its time-dependent changes are extremely small.
As the hydrocarbon adsorbent further deteriorates, it is harder for the hydrocarbon adsorbent to adsorb moisture. Therefore, as the hydrocarbon adsorbent further deteriorates, the hydrocarbon adsorbent adsorbs moisture at a lower rate after its temperature has dropped to a temperature capable of adsorbing moisture. As a result, the maximum value or an increase in the humidity from the time when the internal combustion engine is shut off to the maximum value becomes larger. Furthermore, as the hydrocarbon adsorbent further deteriorates, the amount of moisture absorbed by the hydrocarbon adsorbent until it is saturated becomes smaller. Therefore, when the humidity near the hydrocarbon adsorbent is substantially constant, the humidity is basically higher as the hydrocarbon adsorbent is deteriorated more. Finally, because a gas exchange progresses between the exhaust passage and its exterior (generally, the gas exchange progresses very slowly), the humidity in the exhaust passage is converged to a humidity equivalent to the humidity outside of the exhaust passage. Therefore, the humidity (relative humidity) in the exhaust passage near the hydrocarbon adsorbent after the internal combustion engine is shut off changes in a manner highly correlated to the deteriorated state of the hydrocarbon adsorbent within a period before the humidity is converted to a humidity equal to the humidity in the exhaust passage.
According to the present invention, a method of evaluating a deteriorated state of a hydrocarbon adsorbent which is disposed in an exhaust passage of an internal combustion engine for adsorbing hydrocarbons in an exhaust gas emitted from the internal combustion engine has been devised in view of the above phenomenon. The method comprises the step of, after the internal combustion engine is shut off, evaluating the deteriorated state of the hydrocarbon adsorbent based on an output signal from a humidity sensor which is disposed in the exhaust passage close to the hydrocarbon adsorbent for detecting a humidity in the exhaust passage, within a period of time before the humidity represented by the output signal from the humidity sensor is converged to a humidity outside of the exhaust passage.
In the above method, the output signal from the humidity sensor within the period of time before the humidity near the hydrocarbon adsorbent is converged to the humidity outside of the exhaust passage is used in evaluating the deteriorated state of the hydrocarbon adsorbent. The humidity near the hydrocarbon adsorbent after the internal combustion engine is shut off changes characteristically with respect to the deteriorated state of the hydrocarbon adsorbent as described above. Therefore, the deteriorated state of the hydrocarbon adsorbent can be evaluated based on the output signal from the humidity sensor within the above period of time. Because the internal combustion engine is shut off, the state of the exhaust gas within the exhaust passage including a region near the hydrocarbon adsorbent is unlikely to change instantaneously. Therefore, the output signal from the humidity sensor is stable and highly dependent on the deteriorated state of the hydrocarbon adsorbent. The humidity sensor is not required to be highly responsive insofar as it is capable of detecting a humidity which is unlikely to change instantaneously, i.e., which is highly stable. Therefore, the humidity sensor may be relatively inexpensive. According to the present invention, therefore, the deteriorated state of the hydrocarbon adsorbent can be evaluated highly accurately with a relatively inexpensive arrangement.
According to the present invention, because the humidity near the hydrocarbon adsorbent (the humidity detected by the humidity sensor) changes as described above after the internal combustion engine is shut off, an output signal acquired from the humidity sensor within a predetermined steady period in which the output signal from the humidity sensor is kept substantially constant may be used for evaluating the deteriorated state of the hydrocarbon adsorbent.
The output signal from the humidity sensor in the steady period corresponds to the humidity (detected humidity) near the hydrocarbon adsorbent in a state before the humidity near (around) the hydrocarbon adsorbent is converted to the humidity outside of the exhaust passage after the internal combustion engine is shut off, and also in a state where the humidity is substantially constant (such a state will hereafter be referred to occasionally as xe2x80x9csteady humidity statexe2x80x9d). The output signal from the humidity sensor in the steady period (the output signal is substantially constant in the steady period) depends on the deteriorated state of the hydrocarbon adsorbent, as described above. Since the hydrocarbon adsorbent and the state of the exhaust gas around the humidity sensor near the hydrocarbon adsorbent are substantially static (steady) in the steady period, the output signal from the humidity sensor in the steady period is highly stable and highly reliable as depending on the deteriorated state of the hydrocarbon adsorbent. The deteriorated state of the hydrocarbon adsorbent can thus be evaluated more accurately.
According to the present invention, the steady period comprises, for example, a period after elapse of a predetermined time at least after the internal combustion engine is shut off. Specifically, until the hydrocarbon adsorbent adsorbs moisture therearound and is saturated, i.e., until the steady moisture state is initiated, after the internal combustion engine is shut off, a certain period of time (two to four hours for the exhaust system of an automobile) is needed. According to the present invention, the deteriorated state of the hydrocarbon adsorbent is evaluated based on the output signal from the humidity sensor after elapse of the predetermined time. This allows the output signal from the humidity sensor, which is appropriate for evaluating the deteriorated state of the hydrocarbon adsorbent, to be obtained without the need for monitoring the output signal from the humidity sensor frequently.
According to the present invention, the steady period comprises a period after at least a temperature state close to the hydrocarbon adsorbent is substantially equal to a temperature state outside of the exhaust passage. Specifically, after the internal combustion engine is shut off, the temperature near the hydrocarbon adsorbent drops and is finally converged to a temperature substantially equal to the temperature outside of the exhaust passage. When the temperature state near the hydrocarbon adsorbent is substantially equal to the temperature state outside of the exhaust passage, since the saturated water vapor pressure near the hydrocarbon adsorbent is substantially constant, the humidity near the hydrocarbon adsorbent is also substantially constant. Accordingly, if the steady period comprises the period after the temperature state close to the hydrocarbon adsorbent is substantially equal to the temperature state outside of the exhaust passage, then it is possible to obtain the output signal of the humidity sensor which is appropriate for evaluating the deteriorated state of the hydrocarbon adsorbent.
According to the present invention, the steady period terminates when a predetermined time elapses after the internal combustion engine is shut off. Specifically, after the internal combustion engine is shut off, the humidity near the hydrocarbon adsorbent is finally converged to the humidity substantially equal to the humidity outside of the exhaust passage, as described above. Consequently, the steady period is selected to terminate when the predetermined time elapses after the internal combustion engine is shut off, and the deteriorated state of the hydrocarbon adsorbent is evaluated based on the output signal from the humidity sensor prior to the time when the predetermined time elapses. It is thus possible to obtain the output signal of the humidity sensor which is appropriate for evaluating the deteriorated state of the hydrocarbon adsorbent. The predetermined time referred to above is a time (24 to 72 hours for the exhaust system of an automobile) sufficiently longer than the time when the steady moisture state is initiated after the internal combustion engine is shut off.
According to the present invention in which the output signal from the humidity sensor that is acquired within the steady period is used for evaluating the deteriorated state of the hydrocarbon adsorbent, the deteriorated state of the hydrocarbon adsorbent should preferably be evaluated by comparing either the output signal of the humidity sensor acquired within the steady period or a humidity represented by the output signal with a threshold established depending on the temperature state of the hydrocarbon adsorbent when the output signal of the humidity sensor is acquired.
As described above, the humidity detected by the humidity sensor within the steady period (the humidity near the hydrocarbon adsorbent) is basically higher as the hydrocarbon adsorbent is deteriorated more. Therefore, it is possible to evaluate how the hydrocarbon adsorbent is deteriorated by comparing the output signal acquired from the humidity sensor within the steady period or a humidity represented by the output signal with a suitable threshold. However, the maximum amount of moisture that can be adsorbed by the hydrocarbon adsorbent, and the maximum amount of HC that can be adsorbed by the hydrocarbon adsorbent are affected by the temperature of the hydrocarbon adsorbent. Basically, the maximum amounts of moisture and HC that can be adsorbed by the hydrocarbon adsorbent are greater as the temperature of the hydrocarbon adsorbent is lower. Therefore, the humidity near the hydrocarbon adsorbent in the steady period and the output signal from the humidity sensor are affected by not only the deteriorated state of the hydrocarbon adsorbent, but also the temperature state of the hydrocarbon adsorbent. By setting the threshold to be compared with the output signal from the humidity sensor within the steady period depending on the temperature state of the hydrocarbon adsorbent at the time the output signal from the humidity sensor is acquired, it is possible to evaluate more appropriately the deteriorated state of the hydrocarbon adsorbent based on the output signal from the humidity sensor.
While the temperature state of the hydrocarbon adsorbent which is required to establish the threshold may be directly recognized by a temperature sensor which detects the temperature of the hydrocarbon adsorbent itself, the temperature of the hydrocarbon adsorbent may be estimated from another parameter. Alternatively, temperature data correlated to the temperature state of the hydrocarbon adsorbent (e.g., a detected or estimated value of the engine temperature of the internal combustion engine or the temperature of the exhaust gas near the hydrocarbon adsorbent) may be used as a substitute representing the temperature state of the hydrocarbon adsorbent. The temperature state of the hydrocarbon adsorbent which is to be recognized to establish the threshold is not required to be strictly a temperature state at the same time as when the output signal from the humidity sensor is acquired, but may be a temperature state at any time near that time (or more specifically, any time within a time range in which the temperature state can be regarded as being equal to the temperature state of the hydrocarbon adsorbent at that time).
If the output signal from the humidity sensor within the steady period is used for evaluating the deteriorated state of the hydrocarbon adsorbent, then at least the output signal from the humidity sensor at any one time within the steady period may be used. However the deteriorated state of the hydrocarbon adsorbent may be evaluated using output signals acquired from the humidity sensor at a plurality of times within the steady period.
According to the present invention, the output signal from the humidity sensor which can be used for evaluating the deteriorated state of the hydrocarbon adsorbent is not limited to the output signal from the humidity sensor within the steady period. Specifically, before the humidity detected by the humidity sensor is converged to the humidity outside of the exhaust passage, the deteriorated state of the hydrocarbon adsorbent can be evaluated based on an output signal acquired from the humidity sensor upon elapse of a predetermined time after the internal combustion engine is shut off.
For example, after the internal combustion engine is shut off, the humidity near the hydrocarbon adsorbent (the humidity detected by the humidity sensor) increases up to a maximum value and then decreases to a substantially constant value. The maximum value or a change (increase) in the humidity from the time when the internal combustion engine is shut off to the maximum value depends on the deteriorated state of the hydrocarbon adsorbent. The predetermined time is determined to be close to a time required until the humidity represented by the output signal from the humidity sensor takes a maximum value. With the predetermined time thus established, the deteriorated state of the hydrocarbon adsorbent can be evaluated based on the output signal from the humidity sensor upon elapse of the predetermined time after the internal combustion engine is shut off, without the need for using the output signal from the humidity sensor within the steady period. Inasmuch as the output signal from the humidity sensor in the vicinity of the maximum value, or a change in the humidity from the time when the internal combustion engine is shut off to the maximum value is liable to be affected relatively greatly by differently deteriorated states of the hydrocarbon adsorbent, the deteriorated state of the hydrocarbon adsorbent can be evaluated accurately by using the output signal from the humidity sensor in the vicinity of the maximum value. By using the output signal from the humidity sensor upon elapse of the predetermined time after the internal combustion engine is shut off, the deteriorated state of the hydrocarbon adsorbent can be evaluated without the need for monitoring the output signal from the humidity sensor frequently.
The time when the humidity near the hydrocarbon adsorbent takes a maximum value is affected by the manner in which the temperature of the hydrocarbon adsorbent drops after the internal combustion engine is shut off, e.g., by a rate at which the temperature drops. The manner in which the temperature of the hydrocarbon adsorbent drops is affected by the temperature state of the hydrocarbon adsorbent when the internal combustion engine is shut off and also the temperature state outside of the exhaust passage. Generally, therefore, it takes a longer time for the temperature of the hydrocarbon adsorbent to fall as the temperature of the hydrocarbon adsorbent is higher when the internal combustion engine is shut off. Consequently, the time required until the humidity near (around) the hydrocarbon adsorbent takes a maximum value becomes longer. As the difference between the temperature of the hydrocarbon adsorbent at the time the internal combustion engine is shut off and the ambient temperature is greater, the radiation of heat from the hydrocarbon adsorbent and surrounding parts tends to be accelerated. Therefore, the time required until the humidity near the hydrocarbon adsorbent takes a maximum value becomes shorter.
According to the present invention, the predetermined time should preferably be established depending on the temperature state of the hydrocarbon adsorbent at least when the internal combustion engine is shut off. In particular, the predetermined time should preferably be established depending on the temperature state of the hydrocarbon adsorbent when the internal combustion engine is shut off and a temperature state outside of the exhaust passage.
With the predetermined time thus established, the output signal from the humidity sensor in or nearly in a state where the humidity near the hydrocarbon adsorbent actually takes a maximum value can be used for evaluating the deteriorated state of the hydrocarbon adsorbent. The deteriorated state of the hydrocarbon adsorbent can thus be evaluated more appropriately.
According to the present invention in which the output signal from the humidity sensor that is acquired upon elapse of the predetermined time is used for evaluating the deteriorated state of the hydrocarbon adsorbent, the deteriorated state of the hydrocarbon adsorbent should preferably be evaluated by comparing either the output signal of the humidity sensor acquired upon elapse of the predetermined time or a humidity represented by the output signal with a threshold established depending on the temperature state of the hydrocarbon adsorbent upon elapse of the predetermined time. Alternatively, the deteriorated state of the hydrocarbon adsorbent should preferably be evaluated by storing an output signal acquired from the humidity sensor when the internal combustion engine is shut off as a reference output signal, and comparing either a change from the reference output signal of the output signal acquired from the humidity sensor upon elapse of the predetermined time or a change in a humidity represented by the change in the output signal with a threshold established depending on the temperature state of the hydrocarbon adsorbent upon elapse of the predetermined time.
Specifically, as described above, the maximum value that the humidity detected by the humidity sensor can take, or a change (increase) in the humidity from the time when the internal combustion engine is shut off to the maximum value, depends on the deteriorated state of the hydrocarbon adsorbent. Basically, therefore, it is possible to evaluate the deteriorated state of the hydrocarbon adsorbent by comparing the output signal acquired from the humidity sensor upon elapse of the predetermined time or a change in the output signal from the reference output signal (the output signal from the humidity sensor at the time the internal combustion engine is shut off with a suitable threshold. However, as described above, the maximum amount of moisture that can be adsorbed by the hydrocarbon adsorbent, and the maximum amount of HC that can be adsorbed by the hydrocarbon adsorbent are affected by the temperature of the hydrocarbon adsorbent. Basically, the maximum amounts of moisture and HC that can be adsorbed by the hydrocarbon adsorbent are greater as the temperature of the hydrocarbon adsorbent is lower. By establishing a threshold to be compared with the output signal acquired from the humidity sensor upon elapse of the predetermined time or a humidity represented by the output signal, or to be compared with a change in the output signal of the humidity sensor from the reference output signal or a change in the humidity represented by the change in the output signal, depending on the temperature state of the hydrocarbon adsorbent upon elapse of the predetermined time (at the time the output signal from the humidity sensor for use in evaluating the deteriorated state of the hydrocarbon adsorbent), the deteriorated state of the hydrocarbon adsorbent can be evaluated more appropriately based on the output signal from the humidity sensor upon elapse of the predetermined time.
As is the case with using the output signal from the humidity sensor in the steady period for evaluating the deteriorated state of the hydrocarbon adsorbent, the temperature state of the hydrocarbon adsorbent required to establish the threshold may be a detected value or an estimated value of the temperature of the hydrocarbon adsorbent itself, or temperature data correlated to the temperature state of the hydrocarbon adsorbent may be used as a substitute representing the temperature state of the hydrocarbon adsorbent. The temperature state of the hydrocarbon adsorbent which is to be recognized to establish the threshold is not required to be strictly a temperature state at the same time as the time when the predetermined time elapses (when the output signal from the humidity sensor for evaluating the deteriorated state of the hydrocarbon adsorbent, but may be a temperature state at any time near that time.
The output signal from the humidity sensor often tends to change under the influence of the temperature state of the humidity sensor (or more specifically, the temperature state of an element (sensor body) that is sensitive to the humidity). Therefore, in a situation where the temperature state of the humidity sensor at the time of acquiring the output signal from the humidity sensor (the output signal may hereinafter be referred to as xe2x80x9cdeterioration evaluating output signalxe2x80x9d) for use in evaluating the deteriorated state of the hydrocarbon adsorbent changes depending on operating conditions of the internal combustion engine before it is shut off or conditions such as an ambient condition after the internal combustion engine is shut off, it is preferable to compensate for the effect of a change in the output signal from the humidity sensor on the evaluation of the deteriorated state of the hydrocarbon adsorbent. To make such a compensation, the deteriorated state of the hydrocarbon adsorbent should preferably be evaluated based on the acquired value of the deterioration evaluating output signal of the humidity sensor and the temperature state of the humidity sensor at the time the deterioration evaluating output signal is acquired.
By thus taking into account the temperature state of the humidity sensor at the time the deterioration evaluating output signal is acquired, it is possible to evaluate the deteriorated state of the hydrocarbon adsorbent while compensating for the effect of a change in the output signal from the humidity sensor depending on the temperature state of the humidity sensor. As a result, the result of the evaluation of the deteriorated state of the hydrocarbon adsorbent is made highly reliable.
More specifically, according to the present invention in which the output signal acquired from the humidity sensor within the steady period for evaluating the deteriorated state of the hydrocarbon, the output signal acquired from the humidity sensor within the steady period is corrected depending on the temperature state of the humidity sensor at the time the output signal from the humidity sensor is acquired. With the output signal thus corrected, it is possible to estimate an output signal of the humidity sensor corresponding to a certain constant temperature state of the humidity sensor even if the temperature state of the humidity sensor at the time the output signal thereof is acquired is indefinite. The deteriorated state of the hydrocarbon is estimated by comparing either the corrected output signal of the humidity sensor or a humidity represented by the corrected output signal with a threshold established depending on the temperature state of the hydrocarbon adsorbent at the time the output signal from the humidity sensor is acquired. In this manner, the deteriorated state of the hydrocarbon can appropriately be evaluated without being affected by the temperature state of the humidity sensor.
Even in the case where the output signal from the humidity sensor acquired upon elapse of the predetermined time after the internal combustion engine is shut off is used in evaluating the deteriorated state of the hydrocarbon adsorbent, the effect of a change in the output signal depending on the temperature state of the humidity sensor can be compensated for. In this case, for example, the output signal from the humidity sensor at the time the internal combustion engine is shut off, as corrected depending on the temperature state of the humidity sensor at the time, is stored as a reference output signal. The output signal from the humidity sensor (the deterioration evaluating output signal) upon elapse of the predetermined time after the internal combustion engine is shut off is corrected depending on the temperature state of the humidity sensor at the time. Then, a change in the corrected deterioration evaluating output signal from the reference output signal, or a change in the humidity represented by the change in the corrected deterioration evaluating output, is compared with a given threshold which is established depending on the temperature state of the hydrocarbon adsorbent upon elapse of the predetermined time for thereby evaluating the deteriorated state of the hydrocarbon adsorbent.
For compensating for the effect of a change in the output signal from the humidity sensor depending on the temperature state of the humidity sensor, it is preferable to recognize the temperature state of the humidity sensor before the deterioration evaluating output signal of the humidity sensor is acquired, and, if the recognized temperature state is lower than a first predetermined temperature, heat the humidity sensor with a heater to increase the temperature of the humidity sensor to a temperature equal to or higher than the first predetermined temperature. Specifically, the findings of the inventor of the present invention show that if the temperature state of the humidity sensor is lower than a certain temperature (e.g., 10xc2x0 C.), then the output characteristics of the humidity sensor tend to vary depending on the temperature, resulting in variations in the correlation between the output signal of the humidity sensor and the humidity.
Therefore, if the temperature state of the humidity sensor is lower than the first predetermined temperature (e.g., 10xc2x0 C.), the humidity sensor is heated by the heater to increase the temperature state of the humidity sensor to a temperature state equal to or higher than the first predetermined temperature. In this manner, the deterioration evaluating output signal capable of appropriately evaluating the deteriorated state of the hydrocarbon adsorbent can be acquired reliably from the humidity sensor. As a result, the reliability of the result of the evaluation of the deteriorated state of the hydrocarbon adsorbent is increased, resulting in a better chance to evaluate the deteriorated state of the hydrocarbon adsorbent highly reliably.
According to the present invention, the deterioration evaluating output signal from the humidity sensor may be limited to an output signal acquired from the humidity sensor when the temperature state of the humidity sensor is a temperature state in a range close to a second predetermined temperature. With the deterioration evaluating output signal being thus limited to the output signal acquired from the humidity sensor, since the temperature state of the humidity sensor at the time the deterioration evaluating output signal is acquired is limited to the a temperature in the range close to the second predetermined temperature, the deterioration evaluating output signal can be used as it is in appropriately evaluating the deteriorated state of the hydrocarbon adsorbent without the need for correcting the deterioration evaluating output signal depending on the temperature state of the humidity sensor at the time the deterioration evaluating output signal is acquired. The second predetermined temperature should preferably be equal to or higher than the first predetermined temperature, e.g., in a range from 10 to 30xc2x0 C.
If the deterioration evaluating output signal is limited to an output signal acquired from the humidity sensor when the temperature state of the humidity sensor is a temperature state in the range close to the second predetermined temperature, then it is preferable to recognize the temperature state of the humidity sensor before the output signal from the humidity sensor to be used for evaluating the deteriorated state of the hydrocarbon adsorbent is acquired, and, if the recognized temperature state is lower than the temperature in the range close to the second predetermined temperature, to heat the humidity sensor with a heater to increase the temperature of the humidity sensor to the temperature in the range close to the second predetermined temperature. When the temperature state of the humidity sensor is lower than the temperature in the range close to the second predetermined temperature, the humidity sensor is forcibly heated by the heater to increase the temperature state of the humidity sensor to the temperature state in the range close to the second predetermined temperature. Therefore, there is a better chance to acquire the deterioration evaluating output signal from the humidity sensor in a situation where the temperature state of the humidity sensor is in the range close to the second predetermined temperature, resulting in a better chance to evaluate the deteriorated state of the hydrocarbon adsorbent highly reliably without being affected by the temperature state of the humidity sensor.
According to the present invention, the deteriorated state of the hydrocarbon adsorbent should preferably be evaluated based on the output signal from the humidity sensor after the internal combustion engine is shut off at least under predetermined operating conditions. This allows a state of the exhaust gas (the humidity of the exhaust gas, or the like) in the exhaust passage after the internal combustion engine is shut off to be kept optimum in evaluating the deteriorated state of the hydrocarbon adsorbent. Therefore, the reliability of the evaluation of the deteriorated state of the hydrocarbon adsorbent based on the output signal from the humidity sensor can be increased.
More specifically, the predetermined operating conditions should preferably include a condition relative to an air-fuel ratio before the internal combustion engine is shut off, such that the deteriorated state of the hydrocarbon adsorbent should be evaluated based on the output signal from the humidity sensor if the air-fuel ratio at least immediately before the internal combustion engine is shut off is kept at an air-fuel ratio close to a stoichiometric air-fuel ratio continuously for a predetermined period of time.
Specifically, if the internal combustion engine is operated with the air-fuel ratio thereof being kept as an air-fuel ratio close to the stoichiometric air-fuel ratio, then the exhaust gas emitted from the internal combustion engine contains a relatively large amount of moisture, and the moisture contained in the exhaust gas has a substantially constant concentration. If, therefore, the air-fuel ratio immediately before the internal combustion engine is shut off is continuously kept as an air-fuel ratio close to the stoichiometric air-fuel ratio, then an exhaust gas containing a sufficient amount of moisture at a substantially constant concentration is present near the hydrocarbon adsorbent immediately after the internal combustion engine is shut off. Therefore, the hydrocarbon adsorbent smoothly adsorbs moisture and is saturated thereby, so that a change in the humidity near the hydrocarbon adsorbent depends distinctively on the deteriorated state of the hydrocarbon adsorbent.
Preferably, the predetermined operating conditions include a condition relative to a warmed-up state before the internal combustion engine is shut off, such that the step of evaluating the deteriorated state of the hydrocarbon adsorbent based on the output signal from the humidity sensor is carried out if an engine temperature at least immediately before the internal combustion engine is shut off is equal to or higher than a predetermined temperature. According to such a process, when the internal combustion engine is shut off when it has been warmed up sufficiently, i.e., when the air-fuel mixture is combusted stably in the internal combustion engine, and the hydrocarbon adsorbent has been heated sufficiently to release the moisture that has been adsorbed by the hydrocarbon adsorbent, the deteriorated state of the hydrocarbon adsorbent is evaluated based on the output signal from the humidity sensor. Therefore, any variations in the humidity of the exhaust gas in the hydrocarbon adsorbent immediately after the internal combustion engine is shut off are reduced, and the hydrocarbon adsorbent smoothly adsorbs a maximum amount of moisture depending on the deteriorated state thereof after the temperature of the hydrocarbon adsorbent is lowered to a certain extent. As a consequence, a change in the output signal from the humidity sensor after the internal combustion engine is shut off depends more highly reliably on the deteriorated state of the hydrocarbon adsorbent, thus increasing the accuracy with which to evaluate the deteriorated state of the hydrocarbon adsorbent based on the output signal from the humidity sensor.
According to the present invention, the output signal from the humidity sensor which is used in evaluating the deteriorated state of the hydrocarbon adsorbent is required to be in a state before the humidity near the hydrocarbon adsorbent is converged to the humidity outside of the exhaust passage. According to the present invention, therefore, the exhaust passage should preferably be arranged to hold a space around the humidity sensor in communication with the atmosphere at least through the hydrocarbon adsorbent or a catalytic converter. With the exhaust passage being thus arranged or the humidity sensor being disposed to provide the above layout, ambient air is prevented from entering the space around the humidity sensor by the hydrocarbon adsorbent and a catalytic converter (for purifying the exhaust gas). Therefore, a gas exchange between the space around the humidity sensor (near the hydrocarbon adsorbent) and the exterior is difficult to progress. Thus, the deteriorated state of the hydrocarbon adsorbent can be evaluated appropriately stably. Furthermore, the output signal from the humidity sensor which is capable of evaluating the deteriorated state of the hydrocarbon adsorbent can be obtained for an increased period of time, and the timing to acquire the output signal from the humidity sensor for evaluating the deteriorated state of the hydrocarbon adsorbent is available highly flexibly.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.